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<pre><span class="comment"> /****************************************************************************
 **
 ** Copyright (C) 2006-2008 Trolltech ASA. All rights reserved.
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 ** This file is part of the documentation of the Qt Toolkit.
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 #include &lt;QGraphicsScene&gt;
 #include &lt;QGraphicsSceneMouseEvent&gt;
 #include &lt;QPainter&gt;
 #include &lt;QStyleOption&gt;

 #include &quot;edge.h&quot;
 #include &quot;node.h&quot;
 #include &quot;graphwidget.h&quot;

 Node::Node(GraphWidget *graphWidget)
     : graph(graphWidget)
 {
     setFlag(ItemIsMovable);
     setZValue(1);
 }

 void Node::addEdge(Edge *edge)
 {
     edgeList &lt;&lt; edge;
     edge-&gt;adjust();
 }

 QList&lt;Edge *&gt; Node::edges() const
 {
     return edgeList;
 }

 void Node::calculateForces()
 {
     if (!scene() || scene()-&gt;mouseGrabberItem() == this) {
         newPos = pos();
         return;
     }

     <span class="comment">//</span> Sum up all forces pushing this item away
     qreal xvel = 0;
     qreal yvel = 0;
     foreach (QGraphicsItem *item, scene()-&gt;items()) {
         Node *node = qgraphicsitem_cast&lt;Node *&gt;(item);
         if (!node)
             continue;

         QLineF line(mapFromItem(node, 0, 0), QPointF(0, 0));
         qreal dx = line.dx();
         qreal dy = line.dy();
         double l = 2.0 * (dx * dx + dy * dy);
         if (l &gt; 0) {
             xvel += (dx * 150.0) / l;
             yvel += (dy * 150.0) / l;
         }
     }

     <span class="comment">//</span> Now subtract all forces pulling items together
     double weight = (edgeList.size() + 1) * 10;
     foreach (Edge *edge, edgeList) {
         QPointF pos;
         if (edge-&gt;sourceNode() == this)
             pos = mapFromItem(edge-&gt;destNode(), 0, 0);
         else
             pos = mapFromItem(edge-&gt;sourceNode(), 0, 0);
         xvel += pos.x() / weight;
         yvel += pos.y() / weight;
     }

     if (qAbs(xvel) &lt; 0.1 &amp;&amp; qAbs(yvel) &lt; 0.1)
         xvel = yvel = 0;

     QRectF sceneRect = scene()-&gt;sceneRect();
     newPos = pos() + QPointF(xvel, yvel);
     newPos.setX(qMin(qMax(newPos.x(), sceneRect.left() + 10), sceneRect.right() - 10));
     newPos.setY(qMin(qMax(newPos.y(), sceneRect.top() + 10), sceneRect.bottom() - 10));
 }

 bool Node::advance()
 {
     if (newPos == pos())
         return false;

     setPos(newPos);
     return true;
 }

 QRectF Node::boundingRect() const
 {
     qreal adjust = 2;
     return QRectF(-10 - adjust, -10 - adjust,
                   23 + adjust, 23 + adjust);
 }

 QPainterPath Node::shape() const
 {
     QPainterPath path;
     path.addEllipse(-10, -10, 20, 20);
     return path;
 }

 void Node::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *)
 {
     painter-&gt;setPen(Qt::NoPen);
     painter-&gt;setBrush(Qt::darkGray);
     painter-&gt;drawEllipse(-7, -7, 20, 20);

     QRadialGradient gradient(-3, -3, 10);
     if (option-&gt;state &amp; QStyle::State_Sunken) {
         gradient.setCenter(3, 3);
         gradient.setFocalPoint(3, 3);
         gradient.setColorAt(1, QColor(Qt::yellow).light(120));
         gradient.setColorAt(0, QColor(Qt::darkYellow).light(120));
     } else {
         gradient.setColorAt(0, Qt::yellow);
         gradient.setColorAt(1, Qt::darkYellow);
     }
     painter-&gt;setBrush(gradient);
     painter-&gt;setPen(QPen(Qt::black, 0));
     painter-&gt;drawEllipse(-10, -10, 20, 20);
 }

 QVariant Node::itemChange(GraphicsItemChange change, const QVariant &amp;value)
 {
     switch (change) {
     case ItemPositionHasChanged:
         foreach (Edge *edge, edgeList)
             edge-&gt;adjust();
         graph-&gt;itemMoved();
         break;
     default:
         break;
     };

     return QGraphicsItem::itemChange(change, value);
 }

 void Node::mousePressEvent(QGraphicsSceneMouseEvent *event)
 {
     update();
     QGraphicsItem::mousePressEvent(event);
 }

 void Node::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
 {
     update();
     QGraphicsItem::mouseReleaseEvent(event);
 }</pre>
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